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TITLES IN THE SERIES
BEGjINNIN(3 basic - SPACE ..JOURNEY
GRAPHICS ” HANGMAN
MOVING GRAPHICS -- ALIEN INVADERS
CREATING A DATABASE - ADVENTURE GAME

ISBM 0--531 “03491 “7

A GLOUCESTER PRESS LIBRARY EDITION

First published in
Great Britain in 1985 by
Franklin Watts
12a Golden Square
London W1

First published in the
United States in 1985 by
Gloucester Press

Gopyright © Aladdin Books Ltd 1985

Printed in Belgium

ISBN 0 531 03491 7

Library of Gongress
Gatalog Gard Number: 84-73147

mn ym

Marcus Milton

GLOUCESTER PRESS

NEW YORK ■ TORONTO • 1985

/

F'

Foreword

If you have a computer, then you are almost certain to have
played a Space Invaders game of one kind or another. In this
book, you'll find a program that allows you to create an
"invaders" game of your own, in full color. The program is
given for the Apple He and Commodore 64 computers.

An invaders game relies upon moving graphics to be
effective. The principle is similar to those used in cartoons -
old characters are erased from the screen before being
printed in a new position, giving the impression of
movement. The program has been broken down to its
logical stages, and there is a running text which explains
how each section works. You will find, for example, just how
it is that the computer “knows" when an invader has been
shot by one of your missiles. Writing a program in this way
not only makes it easier for you and other people to
understand, it also makes it run more quickly and reduces
the possibility of errors. At various stages you can test the
sections of program, that you’ve keyed in - even the smallest
error can mean that the program doesn't work. If something
does go wrong check back through the listing very carefully.
Looking for “bugs" in a program is like playing a detective
game - the clues are there for you to spot them.

C#=:.. "

^O PRii^jj'.

>0 X=0:y-J

print T

0 PRiisiT r,

print 77

' '=’PINT Tfi
' '=‘P1NT ta
print TAi

Tftc

!:^PIGHT nrur-

-I^HRt2sscHKt,

"^'*^53CHR:$:2S-

^pscrnt

'"SCHRt,

Contents

Introducing animation

8

User defined characters

10

The flowchart

12

1 THE CONTROL PROGRAM AND INITIALIZATION

13

APPLE He

14

COMMODORE 64

18

2 MOVING AND FIRING

23

'apple He

24

COMMODORE 64

28

3 WINNING AND LOSING

33

APPLE He

34

COMMODORE 64

36

The complete listing

40

Glossary

42

Index

44

Introducing animation

One of the simplest forms of computer animation is achieved
by printing a figure on the screen, erasing it and then
quickly printing the same figure again in a slightly different
position. Repeating this action a number of times gives the
illusion of the figure moving rapidly across the screen. This
short program demonstrates the effect.

o

;!.() HOME

20 FOR ;i;

T‘0

3S

30 VTAB 5s

HTAB

I:; PRINT " X “

40 FDR D

•1

TO

lOOs NEXT D

ilEXT I

u

©

©

The letter X is moved across the screen using a FOR...
NEXT loop, with I as its variable - the changing value of I
gives different screen coordinates for printing “X”. Notice the
space either side of the X. When the letter is moved one
space along to the right, the spaces either side move with it.
The blank space on the left will overprint the old X and
effectively erase it from the screen.

In line 40 a short delay loop is introduced, making the
computer count to 100 (Apple lie) or 200 (Commodore)
before it prints the next “X”. Without the delay loop, the
computer would perform the whole operation so quickly it
would be impossible to follow. Delete line 40 and see what
happens.

;l C) F’R .1 N T " ki.," ; PR I N T' ”

20 FOR 1=1 TO 38
30 PRINTSPC < I) "□ K"-
40 FOR D:=l T O 200;; tie;XT D
50 NEXT 1

8

I OF 'X'

Commodore Sprites

In addition to the move-and-blank technique described
opposite, the Commodore has its own special moving
graphics called sprites.

Sprites do not have to be blanked out and reprinted many
times to make them appear to move. Instead, you will have to
use the special registers in the table below. Later in the
book you will see how, by POKEing numbers into these
registers, sprites for missiles and explosions can be created
and controlled.

Sprites are a complex, but very flexible feature of the
Commodore, and there are many more special sprite
registers. Unfortunately, in this book there isn't room to much
more than show you a small percentage of the possibilities
open to you, but with practise, you will be able to create
stunning animated graphics.

Memory

Location

Name

832

start of sprite data area

53248

X location register

53249

Y location register

53259

sprite enabling register

53264

X register in VIC (video) chip

53269

Y register m VIC (video) chip

53287

color register - sprite one

53289

color register - sprite two

5

9

i

i

i-

j

|j

i.

i

4

I

User defined characters

Games can be made to look far more exciting and
professional by defining your own user defined graphics,
which can be animated using the techniques you will see
later in the book. In the Commodore program you will be
using sprites.

APPLE He

Defining graphics on the Apple involves using shape tables.
These can be very difficult to use until you become familiar
with them. The screen is made up of many dots (or pixels),
and you can instruct your Apple to draw shapes by joining
these dots together. There are eight instructions, numbered
in binary 000 to 111. Instructions 000 to 011 are called move
only operations, which tell the computer to move a pointer
either up, down, left or right one pixel on the screen.

TmLES

move only

(oooooooo)

plot fINP MOVE
4

(oooootoo)

3 ^ ^^^^^(ooooofoO

(OOOOOOf/J ^ ■ JoOOOOOOl)(OOOOOIllj

2 (ooooouo)

(oooooo/o)

TO MEKB N SHPF^

'(OOIIOIOI)

(oaioioi)

(oonono)

With a move only instruction you can move an imaginary
pointer around the screen without drawing anything.
Instructions 100 to 111 are plot and move operations - these
draw as well as move. Starting from a defined point on the
screen, you use the move only instructions to move the
pointer to where you wish to commence drawing, before
using the plot and move instructions to draw it.

The computer doesn’t understand these instructions as they
stand, as 3-bit (binary digit) numbers. They first need to be
collected together into 8-bit numbers, and then converted to
decimal numbers (the kind of numbers humans count in).
Each 8-bit number contains two shape table operations,
usually preceded by two zeroes. You can convert from
binary to decimal by imagining each bit as a column
representing a decimal number - starting from the right, the
columns represent 1, 2, 4, 8, 16, 64, and 128 - and adding the
columns together.

10

lOIOl)

COMMODORE

Computer graphics screens consist of many small dots (or
pixels). Design your sprite on graph paper using a 24- by
21-pixel grid. You can draw any shape you like, shading
squares that you want the computer to color. It understands
that each dot can have the value of 0 or 1. If the dot is set to 1,
then the computer interprets this as a filled dot. If the dot is
set to 0, the computer takes the dot as blank. These Is and Os
are known as binary digits (or bits). There are eight bits to a
byte, and the computer stores each byte in a memory
location. Sprites are defined by POKEing 63 bytes into a
special area of memory. POKE tells the computer to put a
number into a specific memory location.

Sprites can be any size up to 24 by 21 pixels. The diagram
below shows how the missile sprite used later on in the book
5 drawn and how the DATA is calculated.

3 00

60 00
95 00
IS 00
2JS0 0
60 00
111 0 0
in 00^
0 00
0 00
000
0 00
0 00
ooo

Setting up a sprite involves a number of steps, using some
special registers in the Commodore. First, DATA must be
POKEd into an area of memory starting at location 832. The
DATA has to be converted from the eight bit binary
numbers into decimal numbers (the numbers that humans
understand, and programs use) before it can be put into a
program. You can do this by adding up the numbers at the
top of the columns in the diagram above. Up to eight sprites
can be set up at the same time, each needing their own
sprite pointer so the computer knows which sprite you want
to use. Next, the sprite needs to be turned on using the sprite
enehling register (location 53248-1-21). The computer needs
to know what color to make the sprite, so a color code must
be POKEd into the color register (location 53248-1-39 for
sprite 1, location 53248-1-40 for sprite 2, and so on). Finally,
the sprite needs to be positioned on screen, using X and Y
location registers. You will see how the sprites for the missile
and explosion used in the game are set up in this program
later in the book.

11

The flowchart

To plan the Invaders game, it's a good idea to draw up a
flowchart. This is a convenient way of setting out the series of
logical steps that define your program. It consists of different
shaped boxes, representing decisions, actions and so on,
connected by "flowlines". The flowchart is used to create a
CONTROL PROGRAM. This is the core of the program,
which calls up other sections of the program whenever they
are required.

r

'P

erL

flV

■ J

AW

THE CONTROL PROGRAM
AND INITIAUZATION

The control program shows how the
program is structured, using a series of logical
steps called subroutines on the Apple
and Commodore machines. These are grouped together
on the flowchart to show how the control program has
been constructed. The first section
of the program then displays instructions
for the game on the screen.

V O U 1=1 R E 1=1 LONE S P C E P I E O I

PROTECT i: HG T i ^ RF * HHDT R

O EE 14 MOMEN IS VOO *4J EE BE
OTTOCK BV OE IEHS F ROM I HE PEoNE I

H f l n 11 M T S S r O N 1 S T o P R E U E H T O N V

6 O R G O N I iPli H ■ ■ S H i; P E R O M I... O H l> .1. N G .

' y ' Hfl«P E G S1' 1=1 R E I H T E R . l-r E E T

K "" MO U E S * S E H-H R E' 'X- Ei 1*1 I" E. R R 1 O H I

P R E S S S1=' O C E T O F I R E H I! S S :i 1... E

PRESS <=»NV k:ev to EHGOGE ENEHV

By following the flowchart on page 12 and breaking the
program into subroutines placed in a logical order, you can
arrive at the main program for the game, as shown below.
Once you have a main program like this, all you have to do is
write each subroutine in turn.

REM

INVADERS

0

10

GOSUB

8000

REM

INITIALIZE

20

GOSUB

70C)0 s

REM

SCREEN

30

GOSUB

. 1 . 000 :

REM

ALIEN

0

40

GOSUB

2000 s

REM

PL.AYER

50

IF" WIN == 1

OR L..OSr == .1 THEN

0

60

GOTO

30

70

IF WIN == 1

THEN

GOSUB 5000:

o

80

GOSUB

6000:

REM

LOSE

90

END

The first line of a program usually gives its title, by way of a
REMark - see line 5. REMarks help anyone see what is
happening in a program. At the beginning of a game you
should give the player instructions, and set all the variables
to give the player instructions, and set all the variables to
their initial values. Line 10 does this. The colon (:) allows
separate statements to be placed on the same line. The
subroutine which plots the stars and prints the score is called
by line 20.

The next four lines, 30 to 60, form a repeated loop which is
the heart of the program. A randomly selected alien is
moved one position down the screen in line 30. Line 40
allows the player to move and fire a rocket at the aliens. Next
comes an IF... THEN test in Line 50. IF either of these
conditions are true, THEN the computer jumps out of the
loop to line 70. Otherwise, line 60 sends the computer back
to line 30, ready to repeat the move/fire process. WIN and
LOST are simply "flags” which can be set true or false (1 or 0)
to tell the computer how the game has ended. Line 70 tests if
the WIN flag is set. IF it is, THEN the WIN subroutine is
called, using the instruction GOSUB 5000. The computer
GOes TO line 20 for a fresh screen of aliens, and a new
game. If WIN isn’t set, the program calls the LOSE
subroutine, starting at line 6000. All the subroutines follow
later in the book.

The END statement may be placed anywhere that is
convenient. However, it is good practice to place the END at
the end of the control program. This gives a neat finish and
makes the program more readable.

14

BEFOf^E you START
IT tS A GOOD lOEA
TO Pt-AH HOW yOUR
SCREEN WILL LOOK
ON GRAPH PAPER-

The first subroutine called is INITIALIZE (see below). It has
been put at line 8000 because, every time the computer
looks for a subroutine it starts at line 1, looking at each line in
turn. Subroutines which you use frequently should be put at
the start of the program. In line 8010 we clear the text
screen, and bring the cursor to the home position (to the top
left hand corner of the screen) using call TEXT. VTAB means
vertical tab, or move down a certain number of text lines on
screen. HTAB is exactly the same but for horizontal
movement. In line 8020, the computer is instructed to start
PRINTing down two lines, and across seven characters.

S3 >000 REM
801,0 TEXT
8*01::! >L^ 'v I (-1.8
p :i: t...0T"

8030 HTAB
S:i040 HTAB
8050 HTAB
S306'0 i'lrAB
8070 HTAB
8080 HTAB
8090 VTAB
LEFT "

81.'0>0 VTAB

INITIALIZE
; HOME

2s HTAB 7s PRINT "YOU ARE A LONE SPACE

:5s PRINT "PROTECTING THE PLANET EARTH. IN"
5s PRINT "A FEW MOMENTS YOU WILL BE UNDER"
5s PRINT "ATTACK BY ALIENS FROM THE PLANET"
IBs PRINT "VARGON."

5s PRINT "YOUR MISSION IS TO PREVENT ANY"

5s PRINT "VARGONIAN SHIP FROM LANDING."

1.0 5 HTAB 7s PRINT " ' Z ' MOVES STARFIGHTER

1.2 s H T A B 7 s P RIN T " ' X ' M 0 V E S S T A R FIG H T E R

RIGHT"

81.1.0 VTAB 14 5 HTAB 7s PRINT "PRESS SPACE TO FIRE
MISSILE"

81.20 VTAB ISs HTAB Ss PRINT "PRESS ANY KEY TO ENGhGE
ENEMY "5

0:HrTSTAR
= INVADERS

0: XP

> GOSUB 9000

') LOST = OsWIM = Or. SC = Os AH IT
H EI ("j H r == 0 s IM V A D E R '3 “ 10 s A LIE N

) ALIENS == INVADERS _ ..

DI IT X ( INV ADERE3) s DI IT Y ( INVADERS) s DI IT GD"/. (-s-o , 1. X)

FOR A 1 TO INVADERS
D X(A) - A * 3sY(A) = HEIGHT

NEXT A
GET RT
RETURN

As the player is reading the instructions, the computer can
be getting on with initializing the variables used in the game.
In line 8140 a number of flags are set. LOST and WIN are set
to zero since the game has yet to begin. SC represents the
number of aliens that have been shot down. AHIT (alien hit)
and HITSTAR are two flags which check if a missile has
collided with an alien or a star. XP is the position of the
player’s spaceship along the screen’s X-axis. HEIGHT is the
Y position of the space invaders when they start towards the
top of the screen. In Line 8150 the number of ALIENS is set
by INVADERS. This extra variable is used to make it very
easy to change the number of space invaders at the start of
the game. Once INVADERS has been altered, the initial
value of ALIENS will automatically alter throughout the
program. Each space invader is given its own label by
DIMensioning two one-dimensional arrays in line 8160.
DIMX(INVADERS) reserves memory space for the column
position of each space invader, and DIMY(INVADERS) does
the same for the row position. For example, X(2) refers to
the second space invader working from the left of the
screen. GD% is an integer (whole number) array which
holds the location of all the characters. Line 8170 starts a loop
which sets the initial positions of the INVADERS. Line 8180
enters the X and Y position of each invader. The X position is
determined by multiplying the alien’s number by three, and
the Y position by HEIGHT.

s>050

- ^DATA 245,59,63,159,46,1 OS , 73,53^ 63 , UO ,.6^1 +.J , ^^8

„ 46,5,00 , ]. 3,77,13 , ;l. 82,58,223,36,220, 1 4 / , ^8,61 , , K ^ , 1 ^

0 I 3,43, :l. 09,57 „ 4,193,36,4,19::';, 193,32,36,223,256 , ^ / , , .jW

Ul 50 ; :l. 73,191,82,118,182,41,69,41,5,193,193,1 , OO
ih 9060 RETURN _

Five shapes are defined for the game - the missile, the
player's spaceship, the alien, the star and the explosion. The
DATA in lines 9040 and 9050 is POKEd into memory by the
FOR... NEXT loop between lines 9000 and 9030. In the
DATA the first two numbers indicate how many shapes are
being defined (in this case five). The following ten numbers
form five pairs, telling the computer where the data for each
shape begins.

The SCREEN subroutine DRAWs a random background of
stars before the game begins or when the player is skilful
enough to shoot all the aliens. Line 7010 clears the screen
and sets the screen to Page 1 of high resolution graphics
(HGR). In line 7020, POKE - 16301,0 allows you to use the
bottom four lines of the screen for text. POKEing locations
232 and 233 tells the computer where to find the shape table
information for the stars, and SCALE sets their size.

20 RANPOM CHARACTeR
blocks ARe CHOSBN
for THB stars, I
1 WHICH ARB POSITIONBP
AS SHOWN- IF THB
\ ALIBN MOVES INTO
THIS BLOCK 'T WCT
1 NOT PBLBTB THB STAR-

7000
7010
7020

s(::al.e:

7 0^30
7040
9)

7C)50
'7060
7070
7080
■7090
7 :i. 00

REIN S3t:;REEN
l-IGR ; NOME

P 01--; E - :i. 630 ]. , 0 !i P 0 K: EI 232,252: P ORE 2-33,29;

== ].

FOR 3 = 1 TO 20

DF = TNT ( RND (t) * 35):GH = TNT t RND (1) *
GD%(DF,GH) = -J.

HCOI....OR= 7s DRAW 4 AT 8 DF,8 GH

next S

VTAEvi 22 s HTAB 10s PR .TNT "AL. IE.NS DESTROYED- ; SC
HCOLOR- 2s DRAW 2 AT 8 * XP,8 * 19
RETt...lRN

There is a FOR... NEXT loop between lines 7030 and 7070
which plots 20 stars on the screen. Line 7040 uses your
machine's random number generator to decide where to put
the stars. Each time the computer goes through the FOR...
NEXT loop an X and a Y coordinate are chosen. Line 7050
stores the value one in the element in GD% which
corresponds to the star position. Line 7090 DRAWs the
spacecraft at the bottom of the screen, ready to start firing.

By folloAwing the flowchart on page 12 and breaking the
program into subroutines placed in a logical order, you can
arrive at the main program for the game, as shown below.
Once you have a main program like this, all you have to do is
write each subroutine in turn.

5 REM IN'

9ADERS

10

GOSUB

8000 s

REM

INITIALIZE

20

GOSUB

7000 s

REM

SCREEN

30

GOSUB

1000 s

REM

ALIEN

40

GOSUB

2000 s

REM

PLAYER

50

IF WIW=1 OR

LOST

=--.t THEN GOTO 70

60

GOTO

30

7"0

IF WIh4=l THEN GOSUB 5000: GOTO

80

GOSUB

6000 s

REM

LOSE

90

END

20

The first line of a program usually gives its title, by way of a
REMark - see line 5. REMarks help anyone see what is
happening in a program.

At the beginning of a game you should give the player
instructions, and set all the variables to their initial values.
Line 10 does this. The colon (:) allows separate statements to
be placed on the same line. The subroutine which plots the
stars and prints the score is called by line 20. The next four
lines, 30 to 60, form a repeated loop which is the heart of the
program.

A randomly selected alien is moved one position down the
screen in line 30. Line 40 calls a subroutine allowing the
player to move and fire a rocket at the aliens. Next comes an
IF... THEN test in Line 50. IF either of these conditions are
true, THEN the computer jumps out of the loop to line 70.
Otherwise, line 60 sends the computer back to line 30, ready
to repeat the move/fire process. WIN and LOST are simply
"flags" which can be set true or false (1 or 0) to tell the
computer how the game has ended. Line 70 tests if the WIN
flag is set. IF it is, THEN the WIN subroutine is called, using
the instruction GOSUB 5000. The computer immediately
GOes TO line 20 for a fresh screen of aliens, and a new
game. If WIN isn’t set, the program calls the LOSE
subroutine, starting at line 6000.

The END statement may be placed anywhere that is
convenient. However, it is good practice to place the END at
the end of the control program. This gives a neat finish and
makes the program more readable.

BBFOP-B yo<J START
IT IS A GOOD IDEA
TO PLAN HOW YOUR
SCREEN WILL l-OOK
ON GRAPH PAPER ■

The first subroutine is INITIALIZE. Line 8010 sets the screen
colors by POKEmg locations 53280 (the border) and 53281
(the background) with zero (black). POKE 650,255 sets the
keyboard to auto-repeat so the spacecraft moves smoothly,
and the heart-shaped control character clears the screen. Lines
8020 to 8120 contain SPC (SPaCe) commands, to position the
display. The control characters used determine the text color.

___w

8000 REH INITIALIZE _ .

8010 POKE 53280,0: POKE 53281,0: POKE

8020 PRINTSFE(6) "IwVOy ARE A I....OHE
8030 PR INTSPC (4) "PR O T E C T IMG 1 1-1 E P E A H E T
E R Ti"i I

a 040 P RIN T S P C ( 4 ) " A ¥ E IM M O M E M T S V O U 141 L. L
BE OMDER"

8050 PR I N T SPC (4) " A T T A C K Et V A1.... I E M S F R O M
THE PL.AMET"

8080 PR I NTSPC (4) " U A R G O M"

8070 PRINTSPC(4) " VOOR MISSIOM IS TO

fh«e:uemt a MV"

8080 PR I NTSF'C ( 4 ) " U A R Ei O M I A M S H I P FROM
L.AMl> I MG . "

8090 PR I NTSPC (6) " LlL-H Z " MO E S S T A R F 1 G H I E R
8100 Ffa NTSPC (6) " ^ X MO U E S S T « I « »E ^ ” J " f

8110 PRINTSPC(7) "IMPRESS SPAE'E" -JiSaGE

8120 PR I NTSPC (5) " liLElSIP R E S S A M V REV TO ENGAGE

EMEMV"

-O :

L0ST--0; WIM=-Os SC--0: AH I T
H EI e H T --i" IN ‘v A D E S " 1C)

AL. I ENS = I NVADEFvS

DIM X (INVADEr:;S) : DIM Y (INVADERS) s
POP; p,= j. TO IMVADEF-^£)

X A) = A *■ 4 2 s Y (A) “ F-1EI (3 F-l T
GRID';':,(X (A) ,Y(A) )~2
NEXT A
GOSUB 8500
RT-" "

GET RT; IF RT="’' THEN 8220
RETURN

As the player is reading the instructions, the computer can
be getting on with initializing variables. In line 8130 a
number of flags are set. LOST and WIN are set to zero since
the game has yet to begin. SC represents the number of
aliens that have been shot down. AHIT is a flag which is used
to check if a missile has collided with an alien. XP is the
position of the player's spaceship along the screen's
HEIGHT is the Y position of the space invaders. In line 8 14(J
the number of ALIENS is set by INVADERS. If you alter the
value of INVADERS, the initial value of ALIENS will
automatically alter throughout the program. Each invader is
given its own label by DIMensioning two arrays in line 8150.
DIMX (INVADERS) reserves memory space for the column
position of each space invader, and DIMY (INVADERS) does
the same for the row position. Line 8160 starts a FOR...
NEXT loop which sets the initial positions of the INVADERS.
Line 8170 enters the X and Y position of each invader.

HITSTAR=Os XF-20:

DIM GRID"/: (40,21).

,N^C6)

E?.500
8510

8 !:

h:)

8530

8540

8550

8560

8570

8580

REM SPRITE DEFINITION

F(1F; N~-0 TO 62: F;EAD Q: F'UF^E. 8.;'2“t'N, Q: NEXT.

REM MISSILE

PT)R N=0 to 62; F'(EAD Q; PUF=:.E 8V6+N„0; NEXT;

REM EXPLOSION
0=53248
POKE .2040,13
POKE 2041,14
POKE V4-39,2

POKE 04-40,7 . . ^ . ..

DATA 8,0,0,28,0,0,28,0,0,62,0,O,93,O,O,28,0,O,

78,0,0,62,0,0,127,0,0,12/,U,O _ _ _

DATA ' 0,0,0,0,0,0,0,0 , O , 0,0,0,0,0,0,0,0,0,0,0,0,

O „ 0,0,0 , O , 0,0, O , 0,0,0,0 , w cr o 1 Q

data 0,0,0,136,0,17,2,0,64,1 ,0,128,1 «:>, 16o , 8,1 ,

0,129,33,0,132,18,165,72

20

(•36 J. 0 DATA 12 „ 90 48 , J. 8, .16(5,72,0,92,0,0,92,0,72,165,18,
48,90,12

8620 DATA 72,165,18,132,0,33,129,0,129,8,165,16,128,

0,1,64,0,2,17,0,136
8630 f:;e;;tijrn

9000 f-4-i: I NT ” la": !(>l< I I id Y: PF(lNT"ia" 5 : NEIXTs REITURN

On pages 9 and 11 you saw how Commodore sprites are
created and used in theory. This routine sets up the missile
and explosion sprites needed for the game using the
Commodore’s special registers. Line 8510 POKEs the 63
bytes into memory, starting at location 832. Line 8530 makes
V=53248 - this is the lowest numbered memory location and
every higher numbered location only needs be numbered
as V plus a small number. The sprite pointers are set up in
lines 8540 and 8550. Next, the sprite needs to be turned on
using the sprite enabling register (location V+21).

The computer needs to know what color to make the sprite,
so line 8560 POKEs a color code into a color register
(location V+39 for sprite one, location V+40 for sprite 2, and
so on). In this case the code is 2, which is the equivalent to
red sprite graphics.

■0

0

0

7000 REM SCREEN |

7010 POKE V-+-21,Os PR I NT "U" m. |

7020 FOR S=1 TO 40 ^ I

7030 X:= 1 NT ( RND (1 i -i^ ^O) : Y= I NT (RND (2) *21) ; PR I NT " 653” ; |

GOSUB 9000;PRINTTAB(X)"Lg, ” 0 |

'7040 OR I Da < X , Y > =--: 1
7050 NEXT 3

7060 PR :i: NT ” 653" : PR 1 N T SPC (10) " A 8 ... I E 8^ S S> E S T' IS O V E l> ” ; SC
7070 RETI..JRN

Line 20 of the control program instructs the computer to
GOSUB 7000. This subroutine draws a random background of
stars. Line 7010 clears the screen, ready for the background.
There is a FOR... NEXT loop between lines 7020 and 7050
which plots 40 stars on the screen. Line 7030 uses your
machine's random number generator to decide where to put
the stars. Each separate part of an array is called an element,
and a two-dimensional array such as GRID% can be used to
represent a map by making each element correspond to a
grid square on the map. Line 7040 stores the value one in the
element in GRID% which corresponds to the star position
that has just been chosen.

21

At this point it is a good idea to test the program. Follow the
instructions in the box below and you should see the title
screen. Pressing any key will bring up the second screen. If
the program doesn't work, or something peculiar happens,
check through your work very carefully. Even tiny errors
can cause the whole program to crash.

oao! err <■ TVPS A DUMMY LINE 2S B NO A ND HIT RETURN (SO
nHHLt.Ue 2. TYPE RUN AND HIT RETURN CSO

YOU SHOULD NOW SEE THE TITLE SCREEN SHOWN LEFT. PRESSING ANY KEY WILL
ALLOW THE PROGRAM TO CONTINUE, AND THE STAR BACKGROUND WILL BUILD UP,
SHOWN RIGHT. DELETE THE DUMMY LINE 25 BEFORE PROCEEDING TO THE NEXT

SECTION.

YOU ARE A LONE SPACE PILOT
PROTECTING THE PLANET EARTH. IN
A FEW MOMENTS YOU WILL BE UNDER
ATTACK BY ALIENS FROM THE PLANET
VARGON.

YOUR MISSION IS TO PREVENT ANY
VARGONIAN SHIP FROM LANDING.

'Z' MOVES STARFIGHTER LEFT
X' MOVES STARFIGHTER RIGHT
PRESS SPACE TO FIRE MISSILE

PRESS ANY KEY TO ENGAGE ENEMY

A PC/MMY L/A/E a S END THBN H/T
2 Ri/Ai THEN N/T /Ren^

you SHOULD NOW SEE THE TITLE SCf^BEN SHOWN LEFT. PRESSING ANY KEY WILL
ALLOW THE PROGRAM TO CONTINUE, AND THE STAR BACKGROUND WILL BUILD UP,
SHOWN RIGHT. DELETE THE DUMMY LINE :25 BEFORE PROCEEDING TO THE NEXT
SECTION.

In this section of program the aliens move at ranaom aown
and across the screen. It also contains the instructions which
allow the player to move his or her spaceship and to fire
missiles to shoot the aliens down. A running score of the
number of aliens hit is given at the top of the screen.
Good Luck!

The ALIEN subroutine given below has a low number
because it is the most frequently used of the subroutines.

The subroutine moves the aliens. It works by choosing an
alien at random, and blanking it out by DRAWing the
character in black on the screen.

The alien remains invisible until the alien's next position has
been calculated by the computer. Once the new position has
been calculated, the alien is DRAWn on the screen in
yellow. All this happens very quickly and gives the
impression of smooth movement, and works a little like a
cartoon film.

(C)F^

'AlrV

tH?

Pl'

0

1000
1010
THEN
10.20
1030
1040
105C)
1060
107C)
1080
1090
1100
1 1 10

REM ALIEN
R = INT ( RND
RETURN

GD’4 (X (R) ,Y(R) ,)
HCOL.OR== 0; DRAW 3
X(R) = X<R) + INI
Y (R) = Y (R) + 1

(1) *■ INVADERS) -I- 1: IF Y(R)

== GD% (X (R) hYCR) ) 2

AT a * X(R),8 Y(R)

( RND (1) * 3) - 1

IF X(R)
IF X(R)
IF Y(R)
HCOLOR=
GD"/. ( X (R)
RETURN

> 33 THEN X(R) = 33
< 1 THEN X(R) = 1
= 19 THEN LOST ~ 1
5: DRAW 3 AT 8 X(R)

,8 * Y(R)

, Y (R) ) = GD"/. ( X ' R) , Y (R) ) +

Line 1010 selects a random whole number between one and
the value of INVADERS. The value of R identifies a particular
alien. Line 1010 also checks the alien's vertical coordinate. If
it is 22, then it is off-screen, and is “dead” - the subroutine
ends with a RETURN - see the HIT subroutine later on. If a
"living" alien has been chosen, line 1030 DRAWs an alien in
black at Y(R), X(R) making it disappear. New coordinates
are calculated in lines 1040 and 1050, using the RND function.
A random number - either minus one, zero or one - is added
to the current value of X(R) to alter the column position, and
the value of Y(R) is increased by one. The effect of this is to
move the alien's new position one space left and down, or
one space down, or one space right and down. Lines 1060
and 1070 limit the value of X(R), and make sure that the alien
can never be off the screen. The next line, 1080, tests the
value of Y(R). If it is equal to 19, then the alien has reached
the bottom of the screen, and the player has lost. The LOST
flag is therefore set. In line 1090 the alien is DRAWn in
yellow at its new position. Finally, GD% is updated by
adding 2 to the value in the element corresponding to the
alien's X and Y coordinates.

iNr(fiNP(0*3)'

not Tot^ove

TO ^ M&Hr
lUT(liNP0)*3)-l

THB RUEN MOVES C>0\NR\
BECRVSE / /5 RDPEO TO
ns Y(RJ vRi-VE’.'

24

■SOOI^N\
pet) TO

12(3 THEN 2070

2000 FilEM F-'L.AYEF?

201.0 :i; = PEEK ( 1.6384); IF 1

2020 1-4; - CHR-4i: ( ]; - 1.28)

2030 POKE ■" 16368,0

2040 HCOLOR^ 0; DRAW 2 AT 8 XP,19 «• 8

2050 IF (IT == "Z" OR IT = "s") AND XP > 1 THEN XP ^

X F' 1.

2060 IF (IT == "X" OR IT = ">:") AND XP < 33 THEN XP
XP ■+■ 1

2070 HCOLOR" :2; DRAW 2 AT 8 * XP,1V * 8
2080 ‘ IF I == 160 THEN GOSUB 3000
2090 RETURN

The first of the next two subroutines allows you to control the
player's spacecraft, and to fire missiles. The second
subroutine animates missiles and checks for hits.

In line 2010, PEEKing (-16384) looks at the keyboard. The
key press is stored in 1$. The Z key will move the player’s
spacecraft left and the X key moves it to the right.

Line 2040 DRAWs the spacecraft in black. Lines 2050 and
2060 control the movement of the spacecraft. IF the Z key has
been pressed AND the spacecraft's position is greater than
one, THEN the value of XP is decreased by one character
position. Using the < (greater than) and the > (less than)
operators in this way ensures that the spacecraft can never
be offscreen. The expression XP=XP-1 is common in
computer programming - all it means is that the variable is
altered by the amount specified. In this case, XP is
decreased by one. The spacecraft is held in shape number
two and is reDRAWn in purple by line 2070 - this happens
even if the spacecraft hasn’t moved. Line 2080 checks if the
player has pressed the space bar (in which case 1= 160), and
calls the fire routine if appropriate. Finally, line 2090
RETURNS control to the main program.

SP/X£IS

r-x

A

OF p/xees

THE Z KEY MOVES THE
SPACESHIPCA) TO THE
LEFT AS FAR AS COLUMN ±

THE X KEY MOVES THE
SPACESHIP CA) TO THE
RIGHT AS FAR AS COLUMN 17.

■ -

25

Jitaii

300C>

REM FI Rt:

301 0

XM >;p 8

3020

FOR YM = :F7 TO '

303C)

HCOLOR= 2: DRAW

3040

IF (3DX(XP,YM) >

3C>!50

HCOLOR- 0: DRAW

3060

IF GDa(XP,YM)

0 STEF' “ 1
1 AT XM,YM 8
1 THEN C30SUB 4000
:!. A T XM,YM * 8
:1. THEN HCOLOR^^-- 7 s DRAW

YM

30'70
3080

NEXT

RETURN

This routine animates the missile. There is a FOR... NEXT
loop between lines 3020 and 3070, which is set up with
STEP-1. This is a little different from a normal FOR... NEXT
loop because, instead of counting up, the loop counts down
in steps of one. Whene'ver you use a FOR... NEXT loop you
can specify whichever STEP suits your program best. In this
case, starting at YM= 17, the missile is moved up the screen
in steps of one line until it reaches the top line (YM=0). The

missile is DRAWn in purple by line 3030 — HCOLOR—2

means DRAW the shape in purple. In the next line there is a
check to see if that position is already occupied by an alien -
is there a hit? If the value held in GD% for that location is
greater than one, the HIT routine starting at line 4000 is
called. The missile is blanked out in line 3050 by DRAWing it
in black at the same screen position. GD% is checked again
in line 3060, but this time the check is for a star. If the missile
has gone over a star, the star will not reappear unless it is
DRAWn back on screen. Line 3060 takes the star’s
coordinates and DRAWS it in again in white.

XN,0

AT

THE MISSILE I MOVES
UPWARDS AS SHOWN
ON THE LEFT.

O THE ! IS PRINTED.
@ THE REVERSE g
IS PRINTED SO THAT
IT DISAPPEARS.

<D THE ! IS PRINTED
AT THE NEXT
POSITION ABOVE.

THE MISSILE WILL
CARRY ON OP THE
SCREEN UNTIL IT
HAS HIT AN ALIEN
OR HAS REACHED
THE TOP OF THE

screen.

YOU WILL NOT BE
ABLE TO MOVE THE
SPACESHIP OR FIRE
ANOTHER MISSILE
UNTIL THE MISSILE
HAS PISAPPEAREP

R

R

R

'/I

4000

4010

4020

4030

4040

4050

4060

4070

4080

4C>90

4 :l. 00

4 :l. 10

4 :l 20

4 :i. ::>o

4140

REM

FOR A =
IF X';A)
HCOL.OR=
HCOLOR-
HC01....0R==
GDX ■; X (A)

Ftrr
1 TO

0: DRAW
Os DRAW
7: DRAW
Y(A)) =

IMOADERS
XP OR Y<A) <
3 AT 8 *

1 A"r 8 *
5 AT 8 *
(BD/aX <A) ,

X (A)
X (A)

s A = INvADlERS

A

Y(A)

NEXT

ALIEN = ALIEN 1 s SC ■” SC +

‘ IF ALIEN = 0 THEN WIN = 1
VTAB 22s HTAB 27s PRINT SC
HCOLOR- Os DRAW 5 AT 8 *■ Xf
YM = O
RETURN

YM THEN GOTO

4080 ■■ 1

„ 8 * Y < A)

,8 Y(A)

® 1
1

, 8 *■ Y (A)

) - 2

® 1

AHIT 0

® 1

© I

i * YM

® 1

S

The FOR... NEXT loop between lines 4010 and 4080 checks
each of the invaders in turn to see which one has been hit.

The alien which has been hit is DRAWn in black in line 4030,
and the missile is DRAWn in black in line 4040. In line 4050
an explosion is DRAWn in the position where the alien has
been hit. Having destroyed the alien, line 4060 updates
GD%. The alien's position is set to 22 so that it will be
skipped over in line 1010. The loop is exited by setting A
equal to INVADERS. One is subtracted from the number of
ALIENS in line 4090, and the score (SC) increased to give you
a running score display. The AHIT flag is reset to zero. If line
4100 finds that no more ALIENS remain, the WIN flag is set.
The score is displayed by line 4110, and finally the explosion
is DRAWn over in black in line 4120.

TH£ MISSILE MOVES UP

W£ SCPEEN PNP /F/T
oeracTs w rueN pbovf

IT,IT P£&/ST£PS P HIT.
lyE MISSILE RND THE
PUEN PP£ THEN EPflSEJ>
byPE[>RPIM/H 6 THE
CHPRRCTEPS /N BLRCfC.

UN EXPlOSm/5 OPPLVN

IN RNP THE PUEN THEN
HRS ITS VEPPCPL
CXTOPPlNETES MOVEDOFF
SCPEEAl' TV PBROVELT
FROM PLPY.

THE EXPlOSfOY/ /S THEN

R0ROVEO BY REVPRIN/N&

IN BlFtCK.

The ALIEN subroutine given below has a low number
because it is the most frequently used of the subroutines.

The subroutine moves the aliens - represented by a pi sign.

It works by choosing an alien at random, and then blanking it
out by printing the character in black on the screen. The
alien remains invisible until the alien’s next position has been
calculated by the computer. Once the new position has been
calculated, the alien is printed back on the screen in yellow.
All this happens very quickly and gives the impression of
the figure moving rapidly across the screen, and works a
little like a cartoon film.

.1000
:l 010
1 020
1030
;l 040

1050
:l 060
1070
1080
1090
1100
1110
1 120

REM ALIEN

R= I NT (RND (1) K-1NVADERS) +1 s IF Y < R) =22 1 HEN RE I URI'J
Y=Y < R) : X = X (R) : (30S(.JB 9000 5 PR INTSPC ( X > " " ;

GRI m. ( X , Y) =GRID-'l ( X , Y) -"2

IF GR I D>: ( X < R) , Y (R) ) = 1 THEN GOSUB 9000;

PRINTSPC < X)" m . " ;

X (R) = X (R) "I" I NT (RND < 1) *3) 1

Y < R) =Y (R) •+■ 1

IF X(R)>36 THEN X(R)=36
IF X (R)<1 THEN X(R)=1
IF Y<R)=21 THEN L0ST=1
Y“Y(R): X = X(R): G0SUB 90001

PR I NTSPC (X ) "

GR ID7, ( X ,
RETURN

=GRIDX(X,Y)+2

Line 1010 selects a random whole number between one and
the value of INVADERS. The value of R identifies a particular
alien. Line 1010 checks the vertical coordinate of the alien. If
it is 22, then it is off-screen, and is "dead” - the subroutine
ends with a RETURN instruction. What is happening here
will become more clear when the HIT subroutine is
described later on. If a "living alien has been chosen, line
1020 prints a space in black at Y(R), X(R). New coordinates
are calculated in line 1050, using the RND function. A random
number - either minus one, zero or one - is added to the
current value of X(R) to alter the column position, and the
value of Y(R) is increased by one. The effect of this is to
move the alien’s new position one space left and down, or
one space down, or one space right and down. Lines 1070
and 1080 set limits to the value of X(R), and make sure that
the column position can never be off the screen. The next
line, 1090, tests the value of Y(R). If it is equal to 21, then the
alien has reached the bottom of the screen, and the player
has lost. The LOST flag is therefore set. In line 1100 the alien
is printed in yellow at its new position. Finally, GRID /o is
updated by adding 2 to the alien’s X and Y coordinates.

TO MOV£, tSfY*

/NT f3 ^ 1

HOT TO Move j

INT(f^NP(il*3)-l=0

TO Move right:

INT(KN5<li3)~l=*l
TUB ALIEN MOVES

down siNce 1 {^,-

f^POE^ TO THE y(fl^

COORPIN ATE

2000
2010
2020
2030
2040
2050
2060

REM PL.AYER
GET L$

IF I....T=" " THEM GOSUB 3000: REM FIRE

IF LT“"Z" AND XP>1 THEN XP=^=XP-;i.

IF LT="X" AND XP<3'7 THEN XP^=XP+I
Y=22: GOSUB 9000; PR IN T SPC ( XP-1) ” ?! n'ii
RETURN

The first of the next two subroutines allows you to control the
player's ship, and to fire missiles. The player’s ship can move
left and right across the bottom of the screen. The second
subroutine animates missiles and checks for hits.

In real-time games such as this one the player needs to be
able to interact with what is happening on the screen. It is
quite easy to write a routine which allows the player to use
the keyboard to control graphics.

In line 2010 GET looks at the keyboard to see if a key has
been pressed, and stores the result in 1j$. Now you should
test for the keys you are interested in. The next line tests Ij$,
and IF it is a space (the player has pressed the space bar),
THEN the instruction is to GOSUB 3000, which is the missile
firing subroutine - defined below. Lines 2030 and 2040
control the movement of the spacecraft. IF L$ is Z (the Z key
has been pressed) AND the spacecraft's position is greater
than one, THEN the value of XP is decreased by one. Using
the > (greater than) and the < (less than) operators in this
way ensures that the spacecraft can never be offscreen. The
expression XP=XP-1 is common in computer programming
- all it means is that the variable is altered by the amount
specified. In this case, XP is decreased by one. The player’s
spaceship is represented by the letter A, which is PRINTed
on screen in a string of characters (“ a'”), between two blank
spaces. The spaces over-print and hence erase the old
character A when the spaceship is moved left or right.

THE Z KEY MOVES THE
SPACESHIP (A) TO THE
LEFT AS FAR AS
COLUMN 1.

THE X XEV MOVES
THE SPACESHIP CA)
TO THE RIGHT AS FAR
AS COLUMN 37

29

1 STEP -i

POKE V+16,,0

POKE 'v'+lS, :i.

REM FIRE
>;== (XP-K-S) +24
FOR YM~2;l. TO
Y= < YM+8)+50

IF X<256 THEN POKE V,Xs
IF X >255 T" H E N P O K. E V , X -:

POKE 'v'+1,Y2 POKEI V+21,1
IF OR I D'X ( XP , YM) > 1 THEN GOSIJB TOO':..)

NEXT-

POKE V+21,0

. . , RETURN _

■ ■ ■ ■ ■ ' I

The subroutine above is the FIRE routine, which animates
the missile. There is a FOR... NEXT loop with STEP -1
between lines 3020 and 3080, which moves the missile from
the bottom of the screen to the top. The sprite is displayed
on screen using the X and Y location registers and registers
in the VIC (video) chip. In the program these registers are
referred to as V, V+1, V+16 and V+21. Unfortunately, you
cannot POKE the screen coordinates directly into these
registers The value for the X location register is calculated
in line 3010. If X is less than 256 it is POKEd directly into V,
along with zero into V +16, in line 3040. If X is greater than
255 (the largest value a single memory location can contain)
line 3050 subtracts 255 from X. Line 3030 calculates a new
value for the Y location register. Line 3060 POKEs the Y
location register, and switches on the sprite using the sprite
enable register (V+21). A hit on an alien by the missile is
detected in line 3070 and the program is directed to the HIT
subroutine by GOSUB 4000.

©

1

t

WHEN A MISSILE C!)
HITS A CHARACTER
WITH A STAR, THE
STAR IS BLOTTED

outCO. after the

MISSILE HAS been

t

■

_

PELETEP W THE
STAR IS REPRINTETA
IN THE SAME
POSITION (3).

THE MISSII-E WILL
CARHV ON UP THE
SCREEN UNTIL IT
HAS HIT AN ALIEN
OR REACHES THE
TOP OF THE SCREEN.
YOU WILL NOT BE
ABLE TO MOVE THE
SPACESHIP OR FIRE
ANOTHER MISSILE
UNTIL THE MISSILE
HAS PtSAPPEAREP.

30

I

4000 REM HI T'

4010 X=X-S

4020 IF X<256 THEN F^DKE V+2,X!; POKE V+16,0
4030 IF X>255 THEN POKE y42,X“255; POKEV+16,:
4040 POKE yH-3,Ys POKE y+21,2
4050 FOR A=1 TO INVADERS

4060 IF X(A)<>XP OR Y(A)<>YM THEN GOTO 4080
4070 Y (A) =--22 s A= INVADERS
4080 NEXT A

4090 AL.IENS=AL.IENS-1 s SO^^SC+l: AHI T^^^-O

4100 .IF AL.IENS=b THEN WIN==1

4110 Y=YM: GOSUB 9000; PRINTSPC(XP)" "

4120 GRIDX ( XP, YM) ==GR IDX (XP , YM> -2
4130 PR I |■■■JT " H" : PR I NTSPC < 27) " !..g" ; SC
4140 YM=1
4150 RETURN

The routine above displays the explosion sprite on screen
after a direct hit has been scored on an alien. The X and Y
location registers and the two registers on the VIC chip are
used to as they were in the last subroutine. In line 4030,
though, sprite number two is enabled by POKEing V+16
with 2.

After a hit, line 4070 adjusts the alien's position so that it is off
screen - by altering its Y coordinate to 22. Line 4090 alters
the score, and the number of ALIENS remaining, and then
resets the AHIT flag. The alien is blanked out by calling the
subroutine at line 9000 which positions the cursor over the
alien and control RETURNS to the mains program.

<S)

(D

It (

>-

%

k"'

i ir

i Jv

i X

'9

il

I

*

i

1

•• :: A

•* ...• •

i

Sv!

THE WSSILE MOVES
UP flNb IF IT DSTECTS
M ruen/t registers

R HIT. THE RLIEAIS HEIR
CO-ORO/NRTES RRE
REMOVED OFF THE
SCREEN. THE CURSOR
IS POSITIONED TO

overprint theRUEN
RNP THE EXPWStON
SPRITE IS TURNED ON.
WHEN THE FROERRM
RETURNS FRt»i THE
HIT SUBROUTINE,

RLL THE SPRITES
TURNED OFF,

I

A

fps

^ ^ ^

/Si /Sk

Testing your program

You can test the moving and firing sections of the program at
this point before going on to the final chapter of the book.
Follow the instructions given in the boxes below. You should
be able to move your spaceship and attack the oncoming
invaders. As before, if you receive an error message, check
back through your work with the listings given.

ifiPPLEUe

AllEHS DESTROYED^ O

HTi .

Hi ri • irt

/. ryp£ RUN ANP HIT
I ketu^h\

IF everything is correct,

YOU 5HOOLP BB ABLE TO
SHOOT THE ALIENS AS THEY
ADVANCE DOWN THE SCREEN.
THE PROGRAM WILL STOP
IF you KILL THEM ALL, OR
ONE OF THEM REACHES
THE BOTTOM OF THE SCREEN.
DELETE THE DUMMY LINE
BEFORE PROCEEDING.

ignore: the error MESSRGC

PRINTED AT THIS STAee.

COh/IMOPORB

I type "RUN f\NP H/r

\RBri/P.N\

IF everything is correct,

you SHOULD BE TO

shoot the aliens as they

ADVANCE DOWN THE

the program will STOP
IF ONE OF THEM
THE BOTTOM OF THE SCREEN^

delete the dummy line

BEFORE PROCEEDING.
leNOfRE THE ERROR MaS5AG£\
PRINTED AT THIS STAGE.

WINNING
AND LOSING

The winning and losing section is all that remains to make the
game complete. If the player succeeds in shooting all ten
aliens, another wave of attack appears, beginning lower
down the screen and giving the player less time to deal with
the attack. At the end of this section there are some tips to
make the program even more effective, and the program
listing is given in full.

T H E

I N U «=» l> E
H fi U E

I N l> E

HOWEUER.
ilJ i:>ESTROVEl>
<!^E I EN CRU-iET

S' o u

E I OilT <-lOEi :i; N

33

If the player is skilful enough to destroy all the aliens in a
wave, then the WIN flag is set in the HIT subroutine, and the
control program calls the WIN subroutine. If the player lets
the aliens through, then the LOSE subroutine is called
instead.

5C)00

REM

W1IM

5010

TEXT

: HOME

5020

VTAB

12

2 HTAB 12: PRINT "WELL DONE

5030

HTA.B

1

. pRiiMT -this TIME YOU WIN"

5040

HTAB

14

: PI-;INT "NOW PREPARE FOR "

5050

HTAB

14

: PRINT "OUR NEXT ATTACK"

5060

FOR

D

1 TO 1500: NEXT D

5070

Al... I EM

INVADERS: HEIGHT == HEIGHT +

5080

FOR

A

1 TO INVADERS

5090

X (A)

= A

* 3:Y(A) =■ HEIGHT

5100

NEXT

A

5 :l 10

RETI..)RN

EARTHLING"

;WIN 0

Line 5010 clears the screen and sets the cursor to the HOME
position (the top left hand corner of the screen) and
switches to TEXT mode. Lines 5020, 5030 and 5040 PRINT a
“Well Done” message in the top half of the screen, telling the
player that another wave of aliens is on its way. Notice that
each message line is preceded by HTAB so that the
complete message is arranged in the correct place in the
center of the screen. A FOR... NEXT loop at line 5060
introduces a time delay so the player can read the message.

This kind of delay asks the computer to count from one to 1500
- while It is counting it cannot get on with the next part of the
program. If the message was shorter, the computer could be
asked to count to a smaller number (a shorter delay), or if the
message was longer, the computer could be asked to count
to a larger number (a longer delay).

The number of ALIENS is reset to the number of INVADERS
in line 5070, and the WIN flag is switched "off - i.e. becomes
zero - ready for the next wave of aliens. The value of
HEIGHT IS increased by 2, making the next wave of aliens
appear lower down the screen, and increasing the level of
difficulty. The FOR... NEXT loop between line 5080 and line
5100 calculates the X and Y positions of each alien, by
working out the X position from the alien number (A), and
taking the Y position from the HEIGHT value. This is just the
same as was done earlier in the INITIALIZE subroutine. This
latest subroutine can be tested as before, by following the
instructions in the box on the facing page.

6000

FdEM

L.OSE

60 :i. 0

TE.XT

; HOME

6020

VTA.B

b s

HTAB :l.5: PRINT "T H E"

60i::-0

HTAB

12s

PRINT "A L I E N S"

0

6040

HTAB

14

PRINT "H A V E "

t.) y) *3

HTAB

12s

PRINT "L A N D E D”

0

6060

VTAB

12s

HTAB IT: PRINT "HOWEVER”

6070

HTAB

10 s

P R I IM r " Y 0 1...I D E 3 T R 0 Y E D ' '

; SC

6080

HTAB

12s

PRINT "ALIEN CRAFT"

W)

6090

VTAB

20 s

HTAB 4: PRINT "DO YOU

WISH

1

TO F'le^HT

m

AGAIN''
6 :i. 00
6 1 :l. 0
6120

FNIICE
IF AT ==
RETURN

16368,0" SET AT
"Y" OR AT "y"

I'HEI'-J RUN

When the aliens land the LOSE subroutine (above) is called.
The high resolution screen is replaced by the text screen for
the message.

The screen is cleared and a message is PRINTed on the
TEXT screen, telling the player that the invaders have
landed. Next, lines 6060 to 6080 tell the player his or her
score. The semi colon (;) in line 6070 ensures that
the score appears alongside the "YOU ARE DESTROYED”
message. After a pause, the player is given the option of
playing again. If the response is a Y, then the program RUNs
again, otherwise the program RETURNS to the control
program, and ENDs at line 90.

RPPLEUe YOU CAN NOW RUN THE PROGRAM IN

FULL. WHEN THE INVADERS ARE ALL SHOT DOWN, THE
MESSAGE SHOWN LEFT IS DISPLAYED, AND THE GAME
CONTINUES WITH THE ALIENS ADVANCING FROM A
POSITION LOWER DOWN THE SCREEN. IF AN INVADER
REACHES THE BOTTOM OF THE SCREEN, YOU HAVE LOST
AND THE SCREEN WILL DISPLAY THE MESSAGE ON THE RIGHT.

WELL DONE EARTHLING
THIS TIME YOU WIN
NOW PREPARE FOR
OUR NEXT ATTACK

ALIENS DESTR0YED=10

THE

ALIENS
HAVE
L A N,D E D

HOWEVER

YOU DESTROYED 7
ALIEN CRAFT

DO YOU WANT TO FIGHT AGAIN?

35

If the player is skilful enough to destroy all the aliens in a
wave, then the WIN flag is set in the HIT subroutine, and the
control program calls the WIN subroutine. If the player lets
the aliens through, then the LOSE subroutine is called
instead.

Lines 5010, 5020 and 5030 PRINT a “Well Done” message in '
the top half of the screen, telling the player that another
wave of aliens is on its way. Notice that each message line is
preceded by SPC so that the complete message is arranged
in the correct place in the center of the screen. A FOR.. ■
NEXT loop at line 5060 introduces a time delay so the player
can read the message. This kind of delay asks the computer
to count from one to 1500 — while it is counting it cannot get on
with the next part of the program. If the message was
shorter, the computer could be asked to count to a smaller
number (a shorter delay), or if the message was longer, the
computer could be asked to count to a larger number (a
longer delay).

The number of ALIENS is reset to the number of INVADERS
in line 5070, and the WIN flag is switched “off - i.e. becomes
zero - ready for the next wave of aliens. The value of
HEIGHT IS increased by 2, making the next wave of aliens
appear lower down the screen, and increasing the level of
difficulty.

The FOR... NEXT loop between line 5080 and line 5110
repositions the X and Y positions of each alien, by working
out the X position from the alien number (A), and taking the
Y position from the HEIGHT value. This is just the same as
was done earlier in the INITIALIZE subroutine. This latest
subroutine can be tested as before, by following the
instructions in the box on the facing page.

i> e: r s"

REM LOSE
PRIMT"L«"

PRINTSPC(:l.7) "P:1T H EI"

PRINTSPC(:l.2) " I H U A
PR INTSPC (:l. 6) " H A U El"

PRIMTSPC (14) " L. A M ll> E l>"

PR I NT SPC •; 16) '' tiMiLLinH O WE U E R
i“:'R :i: N T SPC (12) " V O E# 8> fil S T ffi!; t:> V E 8>

PR :i: N'T'SPC (14) " A 8 ... I E; 81 AT R A 8- - "I" "

P RIN T S P C (7) ‘' » O V 818 8.1 8^811 S 8-8

A83 A 181" ; INPUT AT
IF' 1....EFTT (AT , 1) = " V " T HEN RUN
RETl..)RN

SC

T 83 S-'■ 183 H T

The final section to be defined is the LOSE subroutine,
starting at line 6000. The screen is cleared and a message is
PRINTed, telling the player that the invaders-have landed.
Next, lines 6060 to 6080 tell the player his or her score. The
semi colon (;) at the end of line 6070 ensures that the score of
the number of aliens killed (SC) appears alongside the "YOU
DESTROYED" message.

The player is given the option of playing the game again
after a pause. Anything the player types is given the label
A$. If the response is any word which starts with a Y, line
6100 RUNS the program again, otherwise the program
RETURNS to the control program, and stops at the END
statement in line 90.

CJOMMOOOne YOU CAN NOW RUN THE. PROGRAM IN FULL.

WHEN THE INVADERS ARE ALL SHOT DOWN, THE MESSAGE
SHOWN left is DISPLAYED, AND THE GAME CONTINUES
WITH THE ALIENS ADVANCING FROM A POSITION LOWER
DOWN THE SCREEN. IF AN INVADER REACHES THE
BOTTOM OF THE SCREEN, YOU HAVE LOST AND THE

II If IIH E F A R I H L I HG

iHjs riMF vnii NIN

NOM I^KFRARF HIR
nilR NFX I A I I AEK

1 H

-1

I N

U A

D t k S

H A

U E ^

I

A N

l> r D

HOME

UFk .

ROS'i D ^

VtHI

l>t G 1

Al

I FN

EkAF 1

DO VOU WTSH 111 ElOIII AGHlN ?

_

37

m

f\

OO 0 C)

Improve your program

As It stands, the Invaders game is fun and challenging to
play. It is difficult to shoot down all the aliens and they
appear to move as if they had a will of their own! However, it
becomes much more like an arcade-style game if you add
some simple sound effects for firing the missiles and alerting
an alien attack. Given below are some hints on how you can
add to the program to get a really professional-looking game
that you and your friends can enjoy.

^ ^ 0 (3

Adding sound

Add these lines to your program to give sound effects - a
movement blip, an explosion noise, and a battle siren.

Both the Apple and the Commodore generate sound effects
when you POKE values into special memory locations. The.
Apple program has a special machine code routine which
plays the sound effects, stored as DATA in line 9070. Every
time a sound effect is needed, a value is POKEd into memory
locations 776 and 777 to set pitch and note length before the
machine code routine is called. The Commodore POKEs
values into registers in a special sound synthesizer chip to
produce its sounds.

APPLE He _

1;}. 05

FhDKE

77ti., I36s POKE 7

...y ..... .. ^

.' I| •...» --^ »

4055

FhDKE

776,254s POKE 7

77 , 1.00

7095

FDR I

j. TO 20 s F"'0H;

.E. / / , ‘

B135

FOR E

; - 776 TO 79£is

READ B

9070

DATA

255,255, 1.73,48

;, 1.92 , :l:

a... •( .1

4

5, 1 74,8,3,76,1.0

, 3 , 96

CALL 778s REM BLIP
i CAL..L- 7 7'8

?0; POKE 777.,305 CAL

)9

Extra Graphics

The graphics additions to the program are used after the
aliens have landed. The Apple program uses the FLASH
command in line 6015 to highlight the message printed in
lines 6020 to 6050. NORMAL in line 6055 stops the following
text from flashing. In the Commodore program the border is
flashed by POKEing the border color location with a range of
color codes.

APPLE He

6015

FLASH

COMMODORE

39

The complete listing

Now that you have the complete program keyed in, you
should store it on tape for future use. The instructions for
saving programs vary slightly between different machines,
and you should consult your user’s manual for detailed
instructions. The full listing for the Space Invaders program
is given below, for both machine versions. The additional
material suggested on the previous page has been included.

APPLE He

COMMODORE

INITIALIZE

SCREEN

ALIEN

PLAYER

1 TMEN

GOTO 70

GOSUB 5000; GOTO 20
LOSE

REM INVADERS
GOSUB 8000: REM
GOSUB 7000; REM
GOSUB 1000: REM
GOSUB 2000; REM
IF WIN = 1 OR LOST
GOTO 30

IF WIN = 1 THEN
GOSUB 6000; REM
END

1000 REM ALIEN

1010 R = INT ( RND (1) ♦ INVADERS) +1: IF Y(R> = 22
THEN RETURN

1020 GD7.(X (R) ,Y(R) ) = GD"/. ( X (R) , Y (R) ) - 2
1030 HCOLOR= 0; DRAW 3 AT 8 * X(R),8 ^

1040 X(R) =-- X<R) + INT ( RND (1) * 3)

1050 Y(R) = Y(R) + 1
1060 IF X(R) > 33 THEN X(R) = 33
1070 IF X(R) < 1 THEN X(R) = 1
1080 IF Y(R) = 19 THEN LOST = 1
1090 HCOLOR= 5; DRAW 3 AT 8 * X(R),8 ■

1100 GD'/. (X <R) ,Y(R) > = GD7.(X (R) ,Y(R) ) + 2
1105 POKE 776,136; POKE 777,30: CALL 778; REM

Y(R)
-- 1

Y(R)

11 10

RETURN

2000

REM PLAYER

2010

I = PEEK ( - 163

84); IF

2020

I^ = CHRT (I - 1

28)

2030

POKE - 16368,0

2040

HCOLOR= 0; DRAW

2 AT 8 +

2050

P - 1

IF (IT = "Z" OR

IT = "z'

2060

IF (IT = "X" OR

IT = ">{’

XP +

1

2070

HCOLOR= 2; DRAW

2 AT 8 H

2080

IF I = 160 THEN

GOSUB :

2090

RETURN

3000

REM FIRE

3010

XM = XP ♦ 8

3020

FOR YM = 17 TO 0 STEP -

3030

HCOLOR= 2; DRAW

1 AT XM

3040

IF GD7. (XP,YM) >

1 THEN

3050

HCOLOR= 0: DRAW

1 AT XM

3060

IF GD7. (XP,YM) =

1 THEN

YM *

8

3070

NEXT

3080

RETURN

4000

REM HIT

4010

FOR A = 1 TO INVADERS

4020

IF X(A) < > XP

OR Y(A)

4030

HCOLOR= 0: DRAW

3 AT a

4040

HCOLOR= 0: DRAW

1 AT 8

4050

HCOLOR= 7: DRAW

5 AT 8

4055

POKE 776,254; POKE 777,

128 THEN 2070

* XP,19 *
) AND XP

< 33 THEN XP

GOSUB 4000

HCOLOR= 7; DRAW 4 AT XM,

< .> YM THEN GOTO 4080

X(A),8 * Y<A)

X (A) ,8 * Y (A)

X(A),8 * Y(A)

4060 GD7.(X(A) ,Y(A) ) = GD7. ( X (A) , Y (A) ) - 2

4070 Y<A) = 22:A = INVADERS
4080 NEXT A

4090 ALIEN “ ALIEN - 1:SC = SC + 1;AHIT - O

4100 IF ALIEN = O THEN WIN = 1

4110 VTAB 22; HTAB 27; PRINT SC

4120 HCOLOR= 0; DRAW 5 AT 8 * XP,8 * YM

4130 YM = 0

4140 RETURN

5000 REM WIN

5010 TEXT ; HOME

VTAB 12: HTAB 12: PRINT "WELL DONE EARTHLING"
HTAB 13; PRINT "THIS TIME YOU WIN"

HTAB 14: PRINT "NOW PREPARE FOR "

HTAB 14; PRINT "OUR NEXT ATTACK"

FOR D = 1 TO 1500: NEXT D
5070 ALIEN = INVADERS: HEIGHT = HEIGHT +■ 2; WIN = O
5080 FOR A = 1 TO INVADERS
5090 X(A) = A * 3;Y(A) = HEIGHT
5100 NEXT A
5110 RETURN

5020

5030

5040

5050

5060

5 REM INVADERS

10 GOSUB 8000: REM INITIALIZE

20 GOSUB 7000: REM SCREEN

30 GOSUB 1000: REM ALIEN

40 GOSUB 2000: REM PLAYER

50 IF WIN=-1 OR L0ST=--1 THEN GOTO 70

60 GOTO 30

70 IF WIN=1 THEN GOSUB 5000; GOTO 20
80 GOSUB 6000; REM LOSE
90 END

1000 REM ALIEN

1010 R=INT(RND(1)*INVADERS)+1; IF Y<R)=22 THEN RETURN
1020 Y=Y(R); X=X(R): GOSUB 9000; PRINTSPC(X)" ";

1030 GRI D“/: ( X , Y) =GR ID7. ( X , Y) --2

1040 IF GRID7.(X (R) ,Y(R) )=1 THEN GOSUB 9000;

PRINTSPC<X) "IJ.";

1050 X(R)=X(R)+INT(RND(1)*3)-1
1060 Y(R>=Y(R)+1
1070 IF X(R>>36 THEN X(R)=36
1080 IF X(R)<1 THEN X(R>=1
1090 IF Y(R>=21 THEN L0ST=1

1100 Y=Y (R) ; X = X ( R > ; GOSUB 9000: , PR INTSPC ( X > " {Tiinr" ;

1105 GOSUB 9100: REM BLIP

1 IT 0 GR ID7. ( X , Y) =GR ID7. ( X , Y) +2

1120 RETURN

2000 REM PLAYER

2010 GET Lt

2020 IF Li:=" " THEN GOSUB 3000; REM FIRE

2030 IF Lt="Z" AND XP>1 THEN XP=XP-1
2040 IF L^="X" AND XP<37 THEN XP=XP+1
2050 Y=22: GOSUB 9000: PRINTSPC (XP-1) " »=*

2060 RETURN

3000 REM FIRE

3010 X=(XP*8)+24

3020 FOR YM=21 TO 1 STEP ~1

3030 Y=<YM*8)-*-50

3040 IF X<25.6 THEN POKE V,X; POKE V+16,0

3050 IF X>255 THEN POKE V,X-255: POKE V+16,1

3060 POKE V-t-l,Y; POKE V+21,1

3070 IF GR ID7. ( XP, YM) > 1 THEN GOSUB 4000

3080 NEXT

3090 POKE V+21,0

3100 RETURN

4000 REM HIT

4010 X=X-8

4020 IF X<256 THEN POKE V+2,X: POKE V+16,0
4030 IF X>255 THEN POKE V+2,X-255: POKEV+16,2
4040 POKE V+3,Y; POKE V+21,2
4045 GOSUB 9200; REM EXPLOSION NOISE
4050 FOR A==l TO INVADERS

4060 IF X(A)<>XP OR Y(A)<>YM THEN GOTO 4080
4070 Y(A)==22; A= INVADERS
4080 NEXT A

4090 ALIENS=AL1ENS-1; SC=SC+1; AH11=0

4100 IF ALIENS=0’THEN WIN=1

4110 Y=YM; GOSUB 9000; PRINTSPC(XP)" "

4120 GR ID7. ( XP , YM) =GR ID7. ( XP , YM) -2

4130 PR I NT " ltd" : PR I NTSPC (27) " \M ''; SC

4140 YM=1

4150 RETURN

5000 REM WIN

5010 PRINT"tJ"

5020 PR I NTSPC (10) " L L. DONE EARTHLING'

5030 PR I NTSPC ( 1 1 ) " L'JT H l S TIME V OIJ WI N "

5040 PR I NTSPC (12)" LlL'IT^aN O W PREPARE FOR "

5050 PR I NTSPC (12)" L'30 UR NEXT A T TACK"

5060 FOR D=1 TO 1500; NEXT D

5070 LET ALIENS=INVADERS; HEIGHT=HEIGHT+2; WIN=0

5080 FOR A=1 TO INVADERS

5090 LET X(A)=A*3; Y(A)= HEIGHT

5100 GRID7.(X(A),Y(A))=2

5110 NEXT A

5120 RETURN

40

I III F 1 C H I

COMMODORE continued

6000 REM lose
6010 PRimfL-"

6020 PRINTSPC(J 7)" I 11 t "

6030 PRINrSPC( 1 " 1. N U r-n D F S"

6040 PRINTSPC< 16) " H rt U F ''

6050 PRINTSPC<14) " I Ft H l> E D"

6060 PR 1 NT SPC (16)" K 3«-« O HE U E: R , "

6070 PRINT3PC< 12) •'veil 1>ES FRUVE I> " ; SC
6080 PR I NTSPC ( 1 4) " A L I I: H C R A F I "

6085 FOR C=1 TO 15

6086 P0KE53280,C

6087 FOR D=1 TO 300:NEXT

6089 NEXT

6090 PR 1 NTSPC (7) " 14.VL‘E'i! JD I) V O U WI
AO A I N" INPUT AT

6100 IF LEFTT(AT,1)=" V " T HEN RUN

6110 RETURN

7000 REM SCREEN

7010 POKE 0+21,0: PRINT" U"

7020 FOR S=1 TO 40

7030 X= I NT (RND (1) *40) : Y= I NT (RND (2) *2.1 ) : PR I ITT " :

G0SUB9000: PR INTTAB(X) " {M . "

7040 f3RID‘4<X , Y)=l
7050 NEXT S

7060 PR 1 NT ■' tfj" : RRI NTSPC ( 10) " A1... 1. E NS l> E S I R O V E l> " ; SC

7065 GOSUB 9300: REM BATTLE SIREN

7070 RETURN

8000 REM INITIALIZE

8010 POKE 53280,0: POKE 53281,0: POKE 650,255:

PRINT"L-"

8020 PR I NTSPC (6) " fc, V OII A R E A ION E S A C E P I I t) T "
8030 PR IN rSPC (4) " F» R O I E C 1 1 N G IMF' PI A N E I

FART FI. IN'

8040 PRINTSPC(4) " A EEW MOMENTS VOU Will BE

oni>e:r"

8050 PR I NTSPC ( 4 > " A I I A C K B V A I 1 E N S FROM I H E
P L A N F T "

8060 PR I NTSPC (4) " U A R G O H "

8070 PR I NTSPC (4) " V O U R MISSION IS 1 O P R F U F N I
ANV"

8080 PR I NTSPC ( 4 ) " U A R G O N I A N S H IF* FROM
LAND ING ."

8090 PR I NTSPC (6) " LLL'jr^ ' Z ^ MO U E S S T A R F I G H T E R

F EF T "

8100 PR I NTSPC (6) " 'X ^ MO U F: S S 1 A R F I G H 1 F: R R 1 G FI T "
8110 PRINTSPC(7) "UlIPRESS SPACE lO FIRE LASER"

8120 PR I NTSPC (5) " L'i'* SP R E S S AN V K E V T O F H G A G E
F NEIMV

8130 L0ST=0: WIN=0; SC=0; AHIT=0: HITSTAR=0; XP=?0:

HEIGHT=1: INVADERB=1O
8140 ALIENS=INVADERS

8150 DIM X(INVADERS): DIM Y(INVADERS):

DIM 6RID>:(40,21 )

8160 FOR A=1 TO INVADERS
81 70 X (A) = A*4--2: Y (A) =HE IGHT
8180 GRID7. ( X (A) , V (A) ) =2
8190 NEXT A
8200 GOSUB 8500
8210

8220 GET RT; IF R;t:="" THEN 8220
8230 RETURN

8500 REM SPRITE DEFINITION r.

8510 FOR N=0 TO 62: READ 0: POKE 832+N,Q: NEXT:

REM MISSILE

8520 FOR N=0 TO 62; READ Q: POKE 896+N,Q: NFXT:

REM EXPLOSION
8530 V=53248
8540 POKE 2040,13
3550 POKE 2041,14
8560 POKE V+39,2
8570 POKE V+40,7

8580 DAT A 8,0,0,28,0,0,28,0,0,62,0,0,93,0,0,28,0,O,

28,0,O,62,0,0,127,0,0,127,0,0
8590 DAT A O,0,0,0,0,0,0,0,0,0,0,0,O,0,0,0,0,0,O 0 O
0,0,0,0,0,0,0,0,0,0,0,0
8600 DATA 0,0,0,136,0,17,2,0,64,1,0,128,16,165,8,
129,0,129,33,0,132,18,165,72
3610 DATA 12,90,48,18,165,72,0,92,0,0,92,0,7^ 165 IS
48,90,12 ’

8620 DATA 72,165,18,132,0,33,129,0,129,8,165,16,
128,0,1,64,0,2,17,O,136
8630 return

9000 PRINT"fea": FOR 1=1 TO Y: PRINT"LU";; NEXT; RETURN
9,100 REM BLIP

911O P0KE54276,0:P0KE54277,0:P0KE54272,0
9120 P0KE54296,15:P0KE54277,128:P0KE54276,’%

9130 P0KE54273,30:P0KE54272,75
9140 RETURN

9200 REM EXPLOSION NOISE

9210 P0KE54283,129;P0KE54284,15:P0KE54280,4o

9220 FOR P=15 TO 0 STEP -1:P0KE 54296,P:NEXT

9230 P0KE54283,0

9240 RETURN

9300 REM BATTLE SIREN

9310 FOR S=1 TO 20

9320 P0KE54276,0:P0KE54277,O:P0KE54272,0
9330 PDKE54296,15; P0KE54277,64;P0KE54276,33
9340 P0KE54273,50:P0KE54272,37
9350 NEXT
9360 RETURN

6000 REM LOSE
6010 TEXT : HOME
6015 FLASH

6020 VTAB 5: HTAB 15: PRINT "T HE"
6030 HTAB 12; PRINT "A L I E N S"
6040 HTAB 14; PRINT "HAVE "

6050 HTAB 12: PRINT "L A N D E D"

605

NORMAL

6060 VTAB 12: HTAB 14; PRINT "HOWEVER"

6070 HTAB 10: PRINT "YOU E^ESTROYED "; SC
6080 HTAB 12; PRINT "ALIEN CRAFT"

6090 VTAB 16; HTAB 4; PRIMf "DO YOU WISH TO FIGHT
AGAIN?"

6100 POKE

16368,0; GET AF
"Y" OR A4 = "y"

THEN RUN

6110 IF A# =

6120 RETURN
7000 REM SCREEN
7010 HGR ; HOME
7020 POKE - 16301,0: POKE 232,252: POKE 233,29; SCAL
E= 1

7030 FOR S = 1 TO 20

7040 DF =INT ( RND (1) * 35):GH =INT ( RND (1) * 1 9)

7050 GD7. (DF,GH) = 1

7060 HCOLOR= 7: DRAW 4 AT 8 * DF,8 * GH
7070 NEXT S

7080 VTAB 22: HTAB 10: PRINT "ALIENS DESTROYED=";SC

7090 HCOLOR= 2: DRAW 2AT8*XP,8*19

7095 FOR I = 1 TO 20: POKE 776,90; POKE 777,30; CALL

778; NEXT

7100 RETURN

8000 REM INITIALIZE

8010 TEXT ; HOME

8020 VTAB 2; HTAB 7: PRINT "YOU ARE A LONE SPACE
PILOT"

8030 HTAB 5: PRINT "PROTECTING THE PLANET EARTH. IN"
8040 HTAB 5; PRINT "A FEW MOMENTS YOU WILL BE UNDER"
8050 HTAB 5: PRINT "ATTACK BY ALIENS FROM THE PLANET"
8060 HTAB 16; PRINT "VARGON."

: PRINT "YOUR MISSION IS TO PREVENT ANY"
PRINT "VARGONIAN SHIP FROM LANDING."

"'Z' MOVES STARFIGHTER
"'X' MOVES STARFIGHTER

8070 HTAB
8080 HTAB

8090 VTAB 10: HTAB 7: PRINT
LEFT"

8100 VTAB 12; HTAB 7: PRINT
RIGHT"

8110 VlAB 14: H1AB 7: PRINT "PRESS SPACE BAR 10 FIRE
MISSILE"

8120 VTAB 18: HTAB 5; PRINT "PRESS ANY KEY TO ENGAGE
ENEMY"

8130 GOSUB 9000

8135 FOR E = 776 TO 798; READ B; POKE E,B; NEXT
8140 LOST = 0:WIN = 0:SC = 0:AHIT = 0;HITSTAR = 0:XP =
16:HEIGHT = 0;INVADERS = 10:ALIEN = INVADERS
8150 ALIENS = INVADERS

8160 DIM X( INVADERS): DIM Y( INVADERS); DIM GD7.(35,19)

8170 FOR A = 1 TO INVADERS

8180 X(A) = A * 3:Y(A) = HEIGHT

8190 NEXT A

8200 GET R4:

8210 RETURN

9000 FOR X = 7676 TO 7804
9010 READ A
9020 POKE X,A
9030 NEXT X

9040 DATA 5,0,12,0,25,0,53,0,80,0,86,0,73,169,63,46,
53,63,46,53,63,22,45,5,O0,73,45,109,58,223,27,55,109,7
3,53,255,219,55,109,73,53,63,63,63,55,54^37,108,73,53,
55,45,00,41,77,9,245,59,63,159,45,45,45,255,219,55,45,
45,45

9050 DATA 245,59,63,159,46,108,73,53,63,00,73,145,58
,46,5,00,13,77,13,182,58,223,36,220,147,46,61,22,109,1
13,43,109,57,4,193,36,4,193,193,32,36,223,255,27,255,5
1,150,173,191,82,118,182,41,69,41,5,193,193,1,00
9060 RETURN

9070 DATA 255,255,173,48,192,136,208,5,206,9,3,240,9
,202,208,245,174,8,3,76,10,3,96

£

i

I

IF

Glossary

Array

DELAY

An array is a set of data, held together and identified by one
variable name (see also the entry for variable). One way of
imagining an array is as a series of boxes within the
computer's memory, with each separate piece of data held
in a separate box.

Delays are sometimes included in computer programs when
it is necessary to slow the computer down. They are usually
part of a FOR... NEXT loop (see below) and look like this in
a program:

FOR DE = 1 TO 1000; NEXT DE.

This would cause the computer to count to 1000 before going
on to the next stage of the program.

DIM The BASIC instruction for opening an array. It is followed by
a number in brackets which tells the computer how big the
array should be.

DRAW Is an APPLE BASIC instruction to DRAW a shape on the
screen. It takes the form DRAW N AT X,Y where N is the
shape table number and X and Y are the screen co¬
ordinates where the shape is to be drawn.

flag A flag is an operator within a program that can be “set" to
either "on" or "off, depending on certain conditions. These
are often used in games to determine whether or not a game
is won or lost - if the game is won, then a “win flag” can be set
from “off, to “on" and the appropriate action is then taken.
Flags are given the values of either 0 or 1, corresponding to
“off and “on" respectively.

GOSUB GOSUB XXXX sends control of the program to a subroutine
starting at line XXXX. The search for line XXXX starts at line
0 - so the program will run faster if subroutines that are
called most often are placed near the start of the listing.

GOTO This instruction tells the computer to go to the Specified line,
missing out any lines in-between. It is used with IF... THEN
(see below). Be careful when using GOTOs, as it’s easy to
have the program jumping backwards and forwards so much
that it is impossible to read.

THEN This is used as a way of telling the computer to do something
only when certain conditions are true. This instruction often
looks something like this; IF score= 10 THEN PRINT “WELL
DONE, YOU'VE WON!!!"

INPUT

This instruction allows the computer to be given information
while a program is running. When the computer comes to an
INPUT instruction it prints a question mark (or, for some
computers, a different symbol) to prompt the user, and waits
for the input to be given.

INT

INT is short for integer, and instructs the computer to make a
whole number of a figure with decimal places in it. It is often
used in conjunction with the RND command which instructs
the computer to generate a random number (see below).

LET

This is one way of giving the computer information. In some
programs there may be statements such as: X= 10

This simply means that the number ten is stored under the
label X. It is often clearer to write:

LET X= 10

The LET statement also gives rise to something that at first
sight seems illogical, if not impossible. In many programs
you will see things like:

LET X=X+1

Of course, in mathematical terms X can't equal X+1. All this
type of statement means is “increase the value of whatever is
stored in X by one."

LIST

This makes the computer display whatever program is has in
its memory. You can LIST single lines, or parts of a program
by following the LIST command with the appropriate line
numbers.

PEEK

Is an instruction to look at the number which is contained
withm a specific memory location given in brackets. For
example PEEK (12345) would look into memory location 12345.

POKE

This instructs the computer to store a piece of information at
a particular memory location. For example, the instruction:
POKE a,b tells the computer to place the information b in
memory location a.

RETURN

This is the signal to end a subroutine. RETURN causes
control of the program to go back to the statement following
the most recently executed GOSUB.

RND

This instruction makes the computer generate a random
number. The precise instruction varies between different
models of computer. Both the Apple He and the Commodore

64 generate random decimal numbers between 0 and 1. To
make whole numbers between a desired range this is
multiplied by a suitable figure and made a whole number
using INT.

43

SPRITE In COMMODORE BASIC a sprite is a high resolution

programmable object that can be made into any shape and
moved about the screen using BASIC instructions. They are
extremely useful for creating smooth animation - you can tell
your sprite to move behind or in front of anything else on the
screen. Even collisions between other sprites can be
detected.

SHAPE TABLE In APPLE BASIC a shape table is a sequence of numbers

that is stored in the computer’s memory. These numbers can
be used by a short routine that creates graphic figures on the
screen by interpreting the numbers as "move only" or “plot
and move" instructions. You can draw the shape anywhere
on the screen using the DRAW instruction.

Index

A

FOR...NEXTloop8,17,21, 26,

R

animation 8

27, 34, 36

random numbers 17, 24, 28, 42

array 16, 20, 21, 42

registers 9,11, 21, 30, 31, 38

G

REM 14, 18

B

graphics 9,11,14,17, 24, 25, 39

RND 24, 42

bit 10,11

graphics grid 16, 21

byte 11, 21

S

I

saving program 40

c

IF...THEN 14,18, 29, 42

score 27, 35, 37

colors 19, 21, 24, 26, 39

integer 16, 42, 43

shape tables 10

control program 12,13, 21, 34,

sound 38

35, 36, 37

L

SPC commands 19, 36

coordinates 17, 24, 27, 28, 30,

loop 16, 26, 27

sprites 9,10,11, 21, 30,31

31, 43

losing 33

subroutine 13

cursor 15, 31, 34

•

M

T

D

memory location 11, 21, 30, 38

testing 22, 32

delay 8, 34, 36

moving 23

text screen 15, 35

F

P

U

firing 23, 33

pixels 10, 24, 28

user-defined characters 10,11

flags 14,16,18, 20, 24, 27, 28, 31,

program, improving 38

34, 36, 42

V

flowchart 12,13

variables 8,14,16,18, 29, 43

44

Design

Cooper • West

Editor

David Rosam

Program editors

Steve Rodgers

Illustrators

Gerard Brown
Andrew Farmer